1. Field of the Invention
The present invention relates to an active matrix (AM)-type flat panel display including thin film transistors, and more particularly, to a flat panel display including polycrystalline silicon active layers and thin film transistors arranged in different directions in sub-pixels.
2. Description of the Related Art
Thin film transistors (TFTs) are used as switching units to control the operations of pixels or as driving units to drive pixels in flat panel displays, such as, liquid crystal displays (LCDs) or organic or inorganic electro-luminescent (EL) displays.
A thin film transistor has a semiconductor active layer, a gate insulating layer formed on the semiconductor active layer, and a gate electrode. In the semiconductor active layer, a drain region and a source region are doped with impurities of a high concentration, and a channel region is formed between the drain and source regions. The gate electrode is formed on the gate insulating layer over the channel region of the semiconductor active layer. The semiconductor active layer can be either an amorphous silicon layer or a polycrystalline silicon layer.
Thin film transistors using amorphous silicon can be deposited at a low temperature, but have degraded electrical characteristics and low reliability, and as such, are generally not suitable for large-sized display devices. Hence, polycrystalline silicon thin film transistors are being used. Polycrystalline silicon has a high mobility of several tens through several hundreds of cm2/V.s, a low high-frequency operating characteristic, and a low current leakage value. Thus, the polycrystalline silicon is very suitable for use in large, high-definition flat panel displays.
As described above, thin film transistors are used, for example, as switching units and/or pixel driving units in flat panel displays. An AM-type organic EL display includes at least two thin film transistors (hereinafter, referred to as TFTs) for each sub-pixel.
An organic EL element has an organic luminescent layer between an anode and a cathode. In the organic EL element, as an anode voltage and a cathode voltage are applied to the anode and the cathode, respectively, holes introduced from the anode are transported to the luminescent layer via a hole transport layer, and electrons introduced from the cathode are transported to the luminescent layer via an electron transport layer. In the luminescent layer, the electrons and the holes are combined to produce exitons. As the excited state of the exitons is changed to a ground state, fluorescent molecules on the luminescent layer emit light to form an image. Generally, in full-color organic EL displays, pixels emitting three colors, namely, red (R), green (G), and blue (B), are included as an organic EL element to achieve a full-color display.
However, in such organic EL displays, R, G, and B luminescent layers emitting R, G, and B light, respectively, have different luminescent efficiency (cd/A) for colors. Since the luminescence of the luminescent layers is approximately proportional to a current value applied to each sub-pixel, for the same current applied to sub-pixels, some colors have low luminescence and some colors have high luminescence. Accordingly, achieving a proper color balance or white balance can be difficult. For example, if the luminescent efficiency of the G luminescent layer is 3 to 6 times higher than those of the R and B luminescent layers, a current 3 to 6 times greater than a current for the G luminescent layer should be applied to the R and B luminescent layers in order to adjust the white balance to a proper level.
A conventional method of adjusting the white balance to a proper level as described above is disclosed in Japanese Patent Publication No. 5-107561, wherein different voltages supplied through a driving line, that is, different Vdd values, are applied to different pixels.
Japanese Patent Publication No. 2001-109399 discloses a method of adjusting a white balance by controlling the size of a driving TFT. More specifically, driving TFTs with different ratios of width W to length L of a channel region are applied to R, G, and B pixels to thereby control the amount of current flowing into each of the R, G, and B organic EL elements.
Japanese Patent Publication No. 2001-290441 discloses a method of adjusting a white balance by forming different sized pixels for the different colors. In this method, a green luminescent region with the highest luminescent efficiency has the smallest luminescent area compared to the red and blue luminescent regions. Thus a proper white balance and a display device with a long life span may be obtained. A different luminescent area can be obtained by varying the anode area.
A method of adjusting luminescence by controlling the amount of current by applying different voltage ranges to R, G, and B color pixels via a data line is also known as a conventional method of adjusting a white balance.
However, none of the methods discussed above consider the crystal structure of a polycrystalline silicon TFT of a flat panel display. In other words, if the direction of arrangement of TFT active layers and the crystal direction of polycrystalline silicon are considered, the mobility may vary according to these directions. In this case, the white balance cannot be adjusted using the above-described methods.
If the amount of current flowing into an organic EL element in each sub-pixel exceeds a limit value, luminescence per unit area is greatly increased by the amount of current greater than the limit value. Accordingly, the life span of the organic EL elements rapidly decreases. Thus, to increase the life span of organic EL elements, an optimal amount of current should to be supplied to each sub-pixel.